The present invention relates to a transmission circuit of a type wherein turning on/off a transmission state is repeated and in particular to a transmission circuit for reducing occurrence of spurious modulation noises at the beginning of the transmission.
A voltage controlled oscillator (VCO) whose oscillation frequency can be controlled according to voltage is used as an intermediate-frequency signal source of a transmission circuit. The VCO is not used singly and is built in a part of a phase locked loop (PLL) for use. The VCO used as an intermediate-frequency signal source of a transmission circuit has an oscillation frequency modulated by a modulation signal. The frequency shift keying (FSK) is one of preferred modulation techniques.
FIG. 3 is a block diagram to show an example of a transmission circuit using a VCO in a related art. In the figure, numeral 1 denotes a fixed-frequency oscillator for transmission, numeral 2 denotes a VCO for outputting a signal containing a modulation signal component, numeral 3 denotes a frequency mixer for mixing (for example, upconverting) an output signal of the fixed-frequency oscillator 1 and an output signal of the VCO 2, and numeral 4 denotes a high-frequency amplifier for amplifying output of the frequency mixer 3 and supplying the amplified signal to a transmission antenna 5. The VCO 2 is built in a PLL 6.
Output of the fixed-frequency oscillator 1 is supplied to the frequency mixer 3 through a narrow-band-pass filter 7. Output of the high-frequency amplifier 4 is supplied to the transmission antenna 5 through a wide-band-pass filter 8. Output of the mixer 3 is supplied to the amplifier 4 through a band-pass filter 9 used as required. An external crystal oscillator and an external loop filter connected to the PLL 6 and an external circuit for inputting a modulation signal to the VCO 2 are not shown.
The transmission circuit in FIG. 3 controls turning on/off power supply +B in two blocks. A first power control block 11 is controlled by a switch SW1 and contains the fixed-frequency oscillator 1, the filter 7, the mixer 3, and the filter 9. A second power control block 12 is controlled by a switch SW2 and contains the amplifier 4.
It is assumed that the transmission circuit in FIG. 3 is not held in a transmission state all the time and that turning on/off the transmission circuit is repeated frequently. At the beginning of the transmission, the switch SW1 is first turned on for first operating the fixed-frequency oscillator 1. Then, oscillation of the oscillator 1 becomes stable before the switch SW2 is turned on for operating the amplifier 4. Therefore, output of the amplifier 4 reaches the maximum amplitude at the same time as the switch SW2 is turned on as shown in a waveform chart in FIG. 3.
If power control is performed in the first and second power control blocks 11 and 12 for sequentially turning on the SW1 and SW2 as in the transmission circuit in the related art shown in FIG. 3, a high-level signal is oscillated suddenly, thus spurious modulation noise occurs. FIG. 4 is a spectrum characteristic chart to show the fact. In the figure, waveform S indicated by the solid line shows the spectrum of a transmission signal of center frequency f0. In contrast, waveform N which is hatched denotes the spurious modulation noise.
The configuration of the transmission circuit in FIG. 3 does not involve any problem if the spurious modulation noise N can be removed through the filter 8 placed at the stage following the amplifier 4. However, since the filter 8 has the bandwidth set wide so as to cover a plurality of channels, noise removal with a limited band cannot be expected and after all, output having a characteristic similar to that in FIG. 4 is transmitted from the antenna 5. This point is the problem to be solved by the present invention.
It is therefore an object of the invention to provide a transmission circuit wherein the spurious modulation noise occurrence level can be remarkably decreased by changing the power control order.
In order to achieve the above object, there is provided a transmission circuit comprising: a fixed-frequency oscillator for oscillating a frequency for the transmission; a voltage controlled oscillator for outputting a signal containing a modulation signal component; a frequency mixer for mixing an output signal of the fixed-frequency oscillator and an output signal of the voltage controlled oscillator; a high-frequency amplifier for amplifying output of the frequency mixer and supplying the amplified signal to a transmission antenna; and a power control circuit for first turning on power of the voltage controlled oscillator and the high-frequency amplifier at the beginning of the transmission and after expiration of a predetermined time, turning on power of the fixed-frequency oscillator.
In the transmission circuit, the power control circuit turns on the power of the fixed-frequency oscillator after the oscillation of the voltage controlled oscillator becomes stable.
In the transmission circuit, the voltage controlled oscillator is built in a phase-controlled loop. Output of the fixed-frequency oscillator is supplied to the frequency mixer through a narrow-band-pass filter. Output of the high-frequency amplifier is supplied to the transmission antenna through a wide-band-pass filter.